There are occasions where an isolated enclosed space must be used even though the atmosphere ambient to the space becomes polluted with contaminants noxious to personnel or damaging to equipment in the space. It is known in such situations to maintain the atmosphere in the space at a pressure higher than ambient, and to supply air to the space through filtering equipment designed to remove the contaminant. It often happens that for long intervals the ambient air is free from contaminants, yet the possibility is present that contaminants may appear on very short notice, and for the sake of security all air admitted to the space is continuously purified. For completeness the purifier must include an adsorptive filter such as one using activated charcoal, and it is well known that for numerous gaseous pollutants the efficacy of activated charcoal is lessened and eventually destroyed by the action of water vapor, present in all ambient atmospheres. Thus even though no specific demand for adsorptive filtering has arisen, filters must nevertheless be periodically discarded and replaced simply as a precautionary measure.
A partial solution lies in retaining the adsorptive filters in the hermetic wrappings in which they are supplied by the manufacturers, but this involves the need to locate the filter, unwrap it, and rapidly install it properly under the stress of emergency conditions, and is not practically acceptable.